Projections by the Energy Information Agency and current Intergovernmental Panel on Climate Change (IPCC) scenarios expect worldwide electric power demand to double from its current level of about 2 terawatts electrical power (TWe) to 4TWe by 2030, and could reach 8-10 TWe by 2100. They also expect that for the next 30 to 50 years, the bulk of the demand of electricity production will be provided by fossil fuels, typically coal and natural gas. Coal supplies 41% of the world's electric energy today, and is expected to supply 45% by 2030. In addition, the most recent report from the IPCC has placed the likelihood that man-made sources of CO2 emissions into the atmosphere are having a significant effect on the climate of planet earth at 90%. “Business as usual” baseline scenarios show that CO2 emissions could be almost two and a half times the current level by 2050. More than ever before, new technologies and alternative sources of energy are essential to meet the increasing energy demand in both the developed and the developing worlds, while attempting to stabilize and reduce the concentration of CO2 in the atmosphere and mitigate the concomitant climate change.
Nuclear energy, a non-carbon emitting energy source, has been a key component of the world's energy production since the 1950's, and currently accounts for about 16% of the world's electricity production, a fraction that could—in principle—be increased. Several factors, however, make its long-term sustainability difficult. These concerns include the risk of proliferation of nuclear materials and technologies resulting from the nuclear fuel cycle; the generation of long-lived radioactive nuclear waste requiring burial in deep geological repositories; the current reliance on the once through, open nuclear fuel cycle; and the availability of low cost, low carbon footprint uranium ore. In the United States alone, nuclear reactors have already generated more than 55,000 metric tons (MT) of spent nuclear fuel (SNF). In the near future, we will have enough spent nuclear fuel to fill the Yucca Mountain geological waste repository to its legislated limit of 70,000 MT.
Fusion is an attractive energy option for future power generation, with two main approaches to fusion power plants now being developed. In a first approach, Inertial Confinement Fusion (ICF) uses lasers, heavy ion beams, or pulsed power to rapidly compress capsules containing a mixture of deuterium (D) and tritium (T). As the capsule radius decreases and the DT gas density and temperature increase, DT fusion reactions are initiated in a small spot in the center of the compressed capsule. These DT fusion reactions generate both alpha particles and 14.1 MeV neutrons. A fusion burn front propagates from the spot, generating significant energy gain. A second approach, Magnetic fusion energy (MFE) uses powerful magnetic fields to confine a DT plasma and to generate the conditions required to sustain a burning plasma and generate energy gain.
Important technology for ICF is being developed primarily at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL), assignee of this invention, in Livermore, Calif. There, a laser-based inertial confinement fusion project designed to achieve thermonuclear fusion ignition and burn utilizes laser energies of 1 to 1.3 MJ. Fusion yields of the order of 10 to 20 MJ are expected. Fusion yields in excess of 200 MJ are expected to be required in central hot spot fusion geometry if fusion technology, by itself, were to be used for cost effective power generation. Thus, significant technical challenges remain to achieve an economy powered by pure inertial confinement fusion energy.
A diode pumped solid-state laser (DPSSL) is a laser that utilizes a solid gain medium, rather than a liquid gain medium, such as in dye lasers, or a gas gain medium, such as in gas lasers. DPSSLs are pumped using one or more diode lasers, also referred to as semiconductor lasers. Generally, the active medium of a DPSSL consists of a glass or crystalline host material that has been doped with a dopant such as neodymium, chromium, erbium, or another suitable ion. Ions of rare earth elements are common dopants for DPSSLs because the excited states of such ions are not strongly coupled with thermal vibrations of the crystalline lattice (phonons) and the lasing threshold can be reached at relatively low pump levels.
Neodymium-doped glass (Nd:glass) and ytterbium-doped glasses and ceramics are used in solid-state lasers at extremely high power (terawatt scale), high energy (megajoules) multiple beam systems for inertial confinement fusion. Titanium-doped sapphire is also widely used for its broad tunability. Diode-pumped solid-state lasers tend to be more efficient that flashlamp pumped systems and have become more common as the cost of high power semiconductor laser pumps has decreased.
Energy scaling and efficiency are important factors in the design and production of next generation diode pumped solid state lasers (DPSSLs). Over the last few decades, gas-cooled slab laser designs have emerged which allow high average power operation in large apertures. One of the difficulties with diode pumping is how to efficiently and homogeneously deliver diode light to the solid state amplifier. In the past, the slab amplifiers have been face-pumped, which creates a homogeneous pump profile, but requires expensive and inefficient diode light transport optics. A preferable technique is to make the diode light delivery system orthogonal to the extraction beam optics in a transverse pumped architecture. Diode light delivered to the edge of the slab will homogenize naturally as the incoherent and divergent diode light experiences total internal reflection inside the slab. However, aperture scaling leads to amplified spontaneous emission, a debilitating loss mechanism, which must be managed through the use of an absorbing perimeter around the slab. This absorbing perimeter typically absorbs diode light as well, which invalidates transverse pumping. The invention employs a geometric technique to manage the amplified spontaneous emission such that transverse pumping is possible along 1-axis. This technique could be employed in a wide variety of slab-like laser or amplifier architectures. A specific example benefiting high energy average power laser systems for inertial fusion energy drivers will be discussed.